GB2143932A

GB2143932A - Furnace

Info

Publication number: GB2143932A
Application number: GB08319824A
Authority: GB
Inventors: Gordon Michael Priest
Original assignee: Individual
Current assignee: Individual
Priority date: 1983-07-22
Filing date: 1983-07-22
Publication date: 1985-02-20
Also published as: GB8319824D0

Abstract

In order to reduce the formation of clinker on the hearth of an incinerator, the hearth has a circulating liquid coolant system with flow channels 1, separated by refractory materials 2, 3, 10, in order to keep the hearth relatively cool. There are porous refractory tiles 3 which pass primary air through the hearth at a relatively low velocity, rather than jetting the primary air in. A flue duct may be arranged for recycling a proportion of the combustion gases and adding them to the primary air supply to the hearth. <IMAGE>

Description

SPECIFICATION Furnace Background of the Invention This invention relates to a furnace, e.g.

acting as an incinerator. The incinerator may.

for instance be for incinerating industrial, hospital and/or municipal waste, and may operate on a continuous or on a batch basis. The waste normally will have sufficient calorific content to maintain combustion, and can be solid hydrocarbon waste, for instance wood, cardboard, paper, plastics materials, fabrics and vegetable matter, and can include for instance metal and glass. Depending on the size of the furnace, the throughput may be from 50 Kg up to 2000-3000 Kg per hour.

The furnace will have a refractory hearth on which solid materials are burnt, and mechanical means such as ploughs can be provided for removing ash from the hearth. Often, there are three separate hearths, each functioning at different temperatures to suit the stage the process has reached, and the material being transported from one hearth to the next hearth using the mechanical means referred to above. The furnace will have a combustion air inlet for introducing combustion air into a bed of ash and combustible material on the hearth and a flue duct for removing combustion gases.

Of particular though not exclusive interest is a furnace of the two-stage or semi-pyrolysis type. In the semi-pyrolysis type, a limited quantity of air is used (e.g. 25% stoichiometric) in the first stage or primary chamber, to produce just enough heat to maintain a semipyrolytic condition; volatiles are removed at temperatures of 600 to 700 C, though this may have to be increased to about 1000"C for some types of waste. Usually, there will be present combustible material in the form of carbon residues which are on the bed of ash directly above the hearth. After this, the gas formed is burnt, normally in a zone of high turbulence, with excess air (say 200% stoichiometric).

Clinker build-up on the hearth, especially on the primary chamber, can be a cause of major shutdown. The clinker adheres to the hearth and may have to be removed by hand. In addition, the clinker removal can cause damage to the hearth. Clinker build-up is caused by the formation of a glassy slag having a low melting point, e.g. of about 850"C, due to the inclusion of various different oxides and the presence of eutectics. If the temperature is sufficiently high, the liquid slag sinks to the bottom of the bed of ash on the hearth, and adheres to the hearth. The hearth itself may be at a high temperature and the molten slag enters voids in the hearth and becomes firmly attached.

Attempts have been made to prevent clinkering by removing all the metal and glass and other unwanted materials from the waste. This is expensive and is not entirely satisfactory as it is not possible to remove smaller items and items which form a part of a larger item (such as nails in wood). Other attempts have been made by adding sand rich in silica to increase the melting temperature of the eutectic formed. The viability of this is dependent upon the amount of sand required and available, the cost of the equipment necessary to feed proportionately to the waste feed, and being able to distribute the sand equally throughout the load.

In operation, the combustion can be started by an outside source, after which air is injected below the bulk of the charge to provide the necessary oxygen to maintain a continuous production of heat. The air is usually injected through a number of jets embedded in or at the side of the hearth. The velocity of air being injected from each jet is comparatively high, such that it penetrates the waste to provide sufficient oxygen to cause a high burning rate in a localised area; this in turn generates very high local temperatures, above the melting point of ash, which melts material and can form clinker. The clinker may grow while leaving areas of unburnt material between the jets.

The Invention, First Aspect According to the first aspect, the invention provides a furnace as set forth in Claim 1.

The circulating liquid, which can be water or any suitable heat transfer liquid, cools the hearth and the arrangement of the flow channels and the rate of flow and temperature of the heat transfer liquid can be such as to chill the slag to a solid state before it reaches the hearth, so that it does not adhere to the hearth. In operation, the actual temperature can be varied by varying the rate of flow of the liquid.

If there is a plurality of hearths, each cooled hearth can be fed from a common liquid supply system and the rate of flow controlled automatically to suit the amount of heat to be removed from the ash present.

The heat can be recovered through a heat exchanger to preheat the air supply to the incinerator and for instance preheat a water feed to a boiler where steam is being produced by the hot gases from the furnace; alternatively, the heat can be transferred to air and blown to waste.

The Invention, Second Aspect According to the second aspect, the invention provides a furnace as set forth in Claim 11.

By inserting porous refractory members into sufficient area of the hearth, the velocity of air can be so low that it prevents any high temperature areas; this results in the pro duction of a fire ash without clinker, which ash is easily removed. In addition, fluidisation of the ash can be avoided with sufficiently low air velocity, and there need be very little carryover of ash.

When the porous refractory members are embedded in the hearth so that there is a depth of ash above them, the air is distributed over the whole area of the hearth, ensuring an even low temperature burn over the whole base area of the load, instead of localised high temperature burns as are produced when using jets This ensures that the whole bottom surface of the load is in a constant state of combustion, instead of localised areas, and pyrolysis can occur over the complete hearth.

In addition, the use of the porous refractory members reduces or prevents unused primary air causing secondary burning of the gases produced in pyrolysis, and the prevention of air leakage past the primary zone keeps the secondary gas temperature lower.

The Invention, Third Aspect According to the third aspect, the invention provides a furnace as set forth in Claim 16.

The combustion or flue gases are mainly carbon dioxide and water vapour. These react endothermically with the carbon in the bed on the hearth, partly to form water gas, and absorb heat from the carbon and from the ash. The combustible material directly on or adjacent the hearth is kept at a low temperature, avoiding the presence of liquid slag on the hearth. Nonetheless the material is later converted into heat, and is not wasted by removal by the mechanical removing means.

To prevent the temperature of the primary chamber of the furnace dropping too far as pyrolytic conditions are reduced, more heat may be required, and this can be provided by an injection of oxygen in the form of air admitted with the flue gas, to burn the carbon. This can be controlled automatically. The carbon burns to carbon monoxide and carbon dioxide, producing high temperatures while the flue gas cools the material above the carbon. The combined temperature of the recycled flue gas and the air to maintain the correct condition can be controlled by the degree of cooling of the flue gas before air is injected into the circuit.

The flue gas can be passed through a boiler (which it can leave at a temperature of say about 240or) before reaching the combustion gas inlet. The flue gas can first be filtered to remove any dust carry-over, and may be cooled by direct or indirect contact with water or by combining these two operations in a single scrubber, after which a fan can inject the flue gas into the furnace.

Combination of Aspects of the Invention Although the invention has been set forth as separate aspects, the advantages of the invention can be enhanced by combining two or three aspects together.

The invention also provides a furnace as set forth in Claim 19 and the method of Claim 20.

The Claims not specifically referred to above claim optional features of the invention.

Preferred embodiments The invention will be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is an isometric projection of a hearth for a multiple-hearth, two-stage incinerator; Figure 2 is a section along the line ll-ll of Figure 1; Figure 3 is a scrap view in the direction of the arrow Ill in Figure 1; Figure 4 shows an alternative to the section of Figure 2; Figure 5 is the view of a hearth of a twostage batch incinerator; Figure 6 is a section along the line VI-VI of Figure 5:: Figure 7 is a view of a hearth of another multiple hearth, two-stage incinerator; Figure 8 is a section along the line VIII-VIII in Figure 7; Figure 9 is a schematic view of another two-stage incinerator; Figure 10 is a schematic view of an incinerator corresponding to that of Figure 9, but not in accordance with the invention; and Figure 11 is a schematic view of yet another multiple hearth, two-stage incinerator.

Figures 1 to 4 The general type of multiple hearth, twostage incinerator is well known, and need not be described in detail. The hearth shown in Figure 1 has a circulating liquid coolant system for abducting heat from the hearth, the coolant system being formed by a serpentine arrangement of flow channels 1 which are spaced apart by cast refractory material 2 (not all of which is shown in Figure 1), porous refractory members or tiles 3, the flow channels 1 being in the form of pipes which are in series. The flow channels 1 are interconnected by suitable interconnecting ducts. Especially in a stepped multiple-hearth incinerator, wear can occur at the ends of the channels 1, and the interconnecting ducts 4 can be below the level of the flow channels 1, as shown in Figure 3. If such accelerated wear occurred at the inlet end of the hearth, a similar arrangement could be made at the inlet end.

The porous tiles 3 can be made for instance of porous carborundum. The porous tiles 3 are connected to ducting 5 beneath the hearth for admitting combustion air through the hearth.

As shown in Figure 1, the porous tiles 3 are positioned between adjacent flow channels 1, and extend substantially from one flow channel 1 to the adjacent flow channel 1. The porous tiles 3 form a relatively small part of the total surface area of the hearth, and are separated by the refractory material 2. Thus the porous tiles 3 may constitute less than 1% of the total surface area of the hearth, but preferably constitute more than 0.35%, the percentage in Figure 1 being about 0.55%.

The flow channels 1 are formed of metal, conveniently steel; the flow channels 1 shown in Figure 1 are of square-section, and their tops form part of the surface of the hearth. In Figure 4, the flow channels 1 are of circular section, but they are in direct contact with and welded to metal plates 6 which form part of the surface of hearth. In general, it is preferred that the surface temperature of the flow channels 1 or metal plates 6 should be from 1 00 C to 1 50 C, this temperature being controlled to avoid condensation of liquids-for instance, to avoid condensation of hydrochloric acid, it is normally considered that one wants a temperature above 1 30 C.

The temperature at the centre parts of the refractory 2 would be in the region of 400"C.

In actual construction, the flow channels 1, 1' rest on a steel plate 7 which is spaced above a base steel plate 8 by joists 9.

Apertured tiles 10 having jet openings 11 are shown in Figures 1 and 2 as alternatives to the porous tiles 3; in one alternative all the tiles are porous and in the other alternative all the tiles are apertured; however, combinations of porous and apertured tiles can be used.

Figures 5 and 6 The two-stage batch incinerator of Figures 5 and 6 has a loader 1 2 and a door 1 3 for the discharge of ash. The hearth is shown as having a number of porous tiles 1 4 carried in solid refractory material 1 5. Below the tiles 14 is ducting 1 6 for admitting combustion air through the tiles 14 and through the hearth.

The hearth of this incinerator does not have a circulating liquid coolant system, and thus the total percentage of the hearth area which is occupied by the porous tiles 1 4 is rather larger, being about 44%. The arrangement of the tiles 14 is only schematic, and the tiles can have any suitable size and any suitable arrangement.

Figures 7 and 8 The multiple hearth, two-stage incinerator of the Figures 7 and 8 has large porous tiles 1 7 which constitute about 50% of the surface area of the hearth. As shown, combustion air is led in through ducting 1 8 beneath the hearth. The remainder of the hearth is formed of impervious refractory 1 9.

Figure 8 illustrates the zones above the hearth, and the temperatures in most of the zones are indicated on the Figures. There is a layer 20 of ash, and, going upwards, a combustion zone 21, a starved air zone 22, a pyrolysis zone 23, a drying zone 24 and a zone 25 containing primary gas. This may be compared with Figure 10, described below.

Figures 9 and 10 Figure 9 is primarily illustrated to compare an incinerator with porous tiles in accordance with the invention with a conventional incinerator. In the incinerator of Figure 9, there are porous tiles 26 which constitute over 50% of the surface area of the hearth, supported above primary air ducting 27 and separated by impermeable refractory 28. In Figure 10, the primary air is jetted in through openings 29. In both Figure 9 and Figure 10, the zones are as referenced in Figure 8. However, in the incinerator of Figure 10, there is a ridge 30 of unburnt material whose base portion is surrounded by a very high temperature zone where the primary air jets create a bunsen burner effect and the temperature on the hearth exceeds the melting point of ash, forming clinker.In addition, cold primary air bypasses the combustible material, as indicated by the arrows 31, and burns on the surface, as indicated at 32. It will be noted that the primary gas temperature is higher than in Figures 8 and 9.

Figure 11 The combustion gas feed arrangment of Figure 11 can be applied to any of the other Figures, as appropriate. There is a multiplehearth, two-stage incinerator having a primary chamber 41 with three hearths 42, 43, 44, and a secondary chamber 45 with a flue duct 46. Waste material is fed in from the right and ploughs act to carry the waste material down over the hearths 42, 43, 44 and to discharge ash from the left. The hearths have porous tiles (which can be as in any of the embodiments above) and there are three combustion air inlets 47 (indicated schematically), leading from a single combustion air duct 48 and each with its own damper or valve control 49 (indicated schematically).The combustion air inlets 47 introduce combustion air into a bed of ash and combustible material resting on the respective hearth 42, 43, 44. The incinerator includes a recycle duct 50 connected to the flue duct 46 for recycling a proportion of the combustion gases via a boiler 51, a filter 52, a spray cooler 53 and a recycle fan 54 to the combustion inlet 48. A recycle temperature control 55 controls the amount of water sprayed into the spray cooler 53. There is an air feed fan 56 which feeds air into the recycled flue gas in an amount controlled by a damper or valve 57 controlled in turn by a primary chamber temperature control 58.

In operation, it is believed that about 1 5 to 20% w/w of the flue gases are recycled, an appropriate percentage of air being added.

It should be noted that the filter 52 and the spray cooler 53 can be combined as a scrubber/cooler.

Claims

1. A furnace having a refractory hearth on which solid materials are burnt, the hearth having a circulating liquid coolant system for abducting heat from the hearth.

2. The furnace of Claim 1, wherein the hearth is formed of flow channels for the circulating liquid, spaced apart by refractory materials.

3. The furnace cf Claim 2, wherein the flow channels are parallel to the ash removal direction.

4. The furnace of Claim 3, wherein, at at least one end of the hearth, one flow channel is connected to the adjacent flow channel by an inter-connecting duct which is below the level of the flow channels.

5. The furnace of any one of the preceding Claims, wherein the hearth also has porous refractory members and ducting for admitting combustion air through the hearth.

6. The furnace of Claim 5 when read as appendant to any one of Claims 2 to 4, wherein the porous refractory members are positioned between adjacent circulating liquid flow channels, extending substantially from one flow channel to the adjacent flow channel.

7. The furnace of Claim 5 or 6, wherein the porous refractory members are separated by imperforate refractory material and by the flow channels, and constitute less than 1% of the total surface area of the hearth.

8. The furnace of any one of Claims 5 to 7, wherein the porous refractory members are separated by imperforate refractory material and by the flow channels, and constitute more than 0.35% of the total surface area of the hearth.

9. The furnace of any one of the preceding Claims, wherein the hearth contains flow channels formed of metal and their tops form part of the surface of the hearth.

10. The furnace of any one of Claims 1 to 8, wherein the hearth has flow channels which are formed of metal and which are in direct contact with metal plates forming part of the surface of the hearth.

11. A furnace having a refractory hearth on which solid materials are burnt, the hearth having porous refractory members and ducting for admitting combustion air through the hearth.

1 2. The furnace of Claim 11, wherein the porous refractory members constitute less than 60% of the total surface area of the hearth.

13. The furnace of Claim 11, wherein the porous refractory members constitute less than 1% of the total surface area of the hearth.

14. The furnace of any one of Claims 11 to 13, wherein the porous refractory members constitute more than 0.35% of the total surface area of the hearth.

1 5. The furnace of any one of Claims 11 to 1 3, wherein the porous refractory members constitute more than 35% of the total surface area of the hearth.

1 6. A furnace having a refractory hearth on which solid materials are burnt, the furnace having a combustion air inlet for introducing combustion air into a bed of ash and combustible material resting on the hearth, and a flue duct for removing combustion gases, the furnace including a recycle duct connected to the flue duct for recycling a proportion of the combustion gases to the combustion air inlet.

1 7. The furnace of Claim 16, and also being in accordance with any one of Claims 1 to 10.

1 8. The furnace of Claim 16, and also being in accordance with any one of Claims 11 to 1 5

1 9. A furnace, substantially as herein described with reference to, and as shown in, any one of Figures 1 to 3, Figure 4, Figures 5 and 6, Figures 7 and 8, Figure 9 and Figure 11 of the accompanying drawings.

20. A method of operating a semi-pyrolytic incinerator, comprising burning solid material in the incinerator of any one of the preceding Claims.

20. A method of operating a furnace, comprising burning solid material in the furnace of any one of the preceding Claims.

1. A semi-pyrolytic incinerator having a refractory hearth on which solid materials are burnt, the hearth having a circulating liquid coolant system for abducting heat from the hearth.

2. The incinerator of Claim 1, wherein the hearth is formed of flow channels for the circulating liquid, spaced apart by refractory materials.

3. The incinerator of Claim 2, wherein the flow channels are parallel to the ash removal direction.

4. The incinerator of Claim 3, wherein, at at least one end of the hearth, one flow channel is connected to the adjacent flow channel by an inter-connecting duct which is below the level of the flow channels.

5. The incinerator of any one of the preceding Claims, wherein the hearth also has porous refractory members and ducting for admitting combustion air through the hearth.

6. The incinerator of Claim 5 when read as appendant to any one of Claims 2 to 4, wherein the porous refractory members are positioned between adjacent circulating liquid flow channels, extending substantially from one flow channel to the adjacent flow channel.

7. The incinerator of Claim 5 or 6, wherein the porous refractory members are separated by imperforate refractory material and by the flow channels, and constitute less than 1% of the total surface area of the hearth.

8. The incinerator of any one of Claims 5 to 7, wherein the porous refractory members are separated by imperforate refractory material and by the flow channels, and constitute more than 0.35% of the total surface area of the hearth.

9. The incinerator of any one of the preceding Claims, wherein the hearth contains flow channels formed of metal and their tops form part of the surface of the hearth.

10. The incinerator of any one of Claims 1 to 8, wherein the hearth has flow channels which are formed of metal and which are in direct contact with metal plates forming part of the surface of the hearth.

11. A semi-pyrolytic incinerator having a refractory hearth on which solid materials are burnt, the hearth having porous refractory members and ducting for admitting combustion air through the hearth.

1 2. The incinerator of Claim 11, wherein the porous refractory members constitute less than 60% of the total surface area of the hearth.

1 3. The incinerator of Claim 11, wherein the porous refractory members constitute less than 1% of the total surface area of the hearth.

1 4. The incinerator of any one of Claims 11 to 13, wherein the porous refractory members constitute more than 0.35% of the total surface area of the hearth.

1 5. The incinerator of Claim 11 or 12, wherein the porous refractory members constitute more than 35% of the total surface area of the hearth.

1 6. A semi-pyrolytic incinerator having a refractory hearth on which solid materials are burnt, the incinerator having a combustion air inlet for introducing combustion air into a bed of ash and combustible material resting on the hearth, and a flue duct for removing combustion gases, the incinerator including a recycle duct connected to the flue duct for recycling a proportion of the combustion gases to the combustion air inlet.

1 7. The incinerator of Claim 16, and also being in accordance with any one of Claims 1 to 10.

1 8. The incinerator of Claim 16, and also being in accordance with any one of Claims 1 1 to 1 5.

1 9. A semi-pyrolytic incinerator, substantially as herein described with reference to, and as shown in, any one of Figures 1 to 3, Figure 4, Figures 5 and 6, Figures 7 and 8, Figure 9 and Figure 11 of the accompanying drawings.